The present invention generally relates to a ferrodielectric substance thin film. The present invention particularly relates to a ferrodielectric substance thin film which is formed on a monocrystal substrate by use of the hydrolysis of an organic metal compound, which has a surface optically smooth and transparent, and which has orientation property so that it can be used as an optical element, and relates to a method of producing such a ferrodielectric substance thin film.
Thin films of ferrodielectric substance have various characteristics proper to ferrodielectric substances, such as ferrodielectricity, piezoelectricity, pyroelectricity, electro-optic effects, and so on so that it has been intended to apply ferrodielectric substance thin films to nonvolatile memories, surface elastic wave elements, infrared-ray pyroelectric elements, acoustooptic elements, electro-optic elements, and so on. Of those applications, for the application to electro-optic elements having a thin film optical waveguide structure, such as secondary higher harmonic elements, optical modulation elements, and so on, it is necessary to reduce losses of light and improve characteristics correspondingly to those of a monocrystal, so that it is inevitable to produce a monocrystal thin film. It has been therefore general to form an epitaxial ferrodielectric substance thin film of BaTiO.sub.3, PbTiO.sub.3 Pb.sub.1-x La.sub.x (Zr.sub.1-y Ti.sub.y).sub.1-x/4 O.sub.3 (PLZT), LiNbO.sub.3, KNbO.sub.3, Bi.sub.4 Ti.sub.3 O.sub.12, Sr.sub.1-x Ba.sub.x Nb.sub.2 O.sub.2, etc. onto an oxide monocrystal substrate by a method such as rf-magnetron sputtering, ion beam sputtering, pulsed laser deposition, MOCVD, etc. However, each of these methods has problems on control of compositions or surface characteristics of a thin film as well as high cost of the equipments, and comparative high temperature not lower than 500.degree. C. is required as temperature of growth.
There is also a method of obtaining a ferrodielectric substance thin film with an organic metal compound by use of hydrolysis thereof, as disclosed in Japanese Patent Examined Publication No. Sho 62-27482. In this method, there are advantages from various views such as accurate control of chemical compositions, uniform level of molecules, lowered temperature in the process, large area of an element, low o cost of equipments, and so on. However, only a multicrystal thin film of a low density (refer to FIG. 1) can be obtained if baking is performed at high temperature, so that the physical property based on the polarization of a ferrodielectric substance cannot be utilized enough, and scattering of light due to grain boundaries and pin holes is so large that it cannot be used as an optical waveguide or the like. In FIG. 1, the reference numeral 1 represents a monocrystal substrate, and 4 represents a multicrystal thin film.
The present inventor has found that a monocrystal ferrodielectric substance thin film can be formed on a monocrystal substrate by epitaxial growth if an organic metal compound which is not hydrolytic is used (K. Nashimoto and M. J. Cima: "Epitaxial LiNbO.sub.3 Thin Films Prepared by a Sol-Gel Process", Mater. Lett., 10, 7, 8 (1991) 348.).
However, according to this method, a ferrodielectric substance thin film baked at a temperature of about 400.degree. C. seems a monocrystal and has an optically smooth surface, but includes minute holes of diameter of several nm, so that the density is not enough high, and the refractive index is not equivalent to that of a monocrystal. A ferrodielectric substance thin film baked at a temperature of about 700.degree. C. is like a monocrystal and has an extremely large sub-grain structure (a grain-like structure, but the directions of respective grains are almost or perfectly constant (refer to FIG. 2)) in comparison with a multicrystal film or an oriented film, and has a density which is high and a refractive index which is equivalent to a monocrystal, but the surface is not smooth optically because of the sub-grain structure, and the transparency of the film is low. In FIG. 2, the reference numeral 1 represents a monocrystal substrate, and 2 represents an epitaxial ferrodielectric substance layer having a sub-grain structure obtained by high temperature baking.